Question

Which alcohol immediately forms 2 layers with the Lucas test?

Answer 1

Lucas test

1. Lucas Test: In this test, Alcohol is treated with an equimolar mixture of concentrated Hydrochloric acid $\left(\mathrm{HCl}\right)$ and anhydrous Zinc Chloride$\left({\mathrm{ZnCl}}_2\right)$ (Lucas reagent).
2. In this reaction, Alcohol gets converted to Alkyl halide. As Alkyl Halides are insoluble in water, their formation is indicated by the appearance of turbidity (formation of the layer) in the reaction mixture.
3. The reactivity order of Alcohols with halo acids is$3°>2°>1°$.
4. As tertiary alcohols are highly reactive they form turbidity immediately.
5. Secondary alcohols form turbidity after 5 minutes.
6. Primary alcohols form turbidity only upon heating.

The reaction involves in the Lucas test

1. It is a type of ${\mathrm{SN}}_1$Nucleophilic substitution reaction. The mechanism involves two steps

Step1-Protonation of Alcohol- The Alcohol gets protonated by the$\mathrm{HCl}$, Now Chlorine is a better nucleophile than water so it will replace water and leads to the formation of Carbocation.

Step2-Chlorine will attack Carbocation and leads to the formation of Alkyl halide.

Thus, tertiary alcohol forms an oily layer immediately with the Lucas test.